1. Field of the Invention
The present invention relates generally to heating devices and more particularly, to a heating device using a photodetector to detect temperature and a method for protecting the same.
2. Description of the Related Art
A conventional heating device is primarily composed of a housing having a flow channel therein and a heater disposed in the flow channel. When the heating device is in operation, working fluid, which is usually air or inert gas, is introduced into the housing from an entrance of the housing, and then discharged out of the housing from an outlet of the housing through the flow channel. The temperature of the working fluid is raised when the working fluid flows past the heater and takes away the heat generated by the heater. In this way, the heating device can discharge high-temperature working fluid to heat up an object located outside the outlet of the housing.
A temperature sensor such as a thermocouple, a bimetal thermostat and so on is usually disposed around the outlet of the heating device for detecting the temperature of the working fluid discharged by the heating device. The signal of the temperature sensor is sent to a controller for enabling the controller to control the operation of the heater according to the temperature of the working fluid discharged by the heating device. However, if the outlet or the entrance of the heating device is blocked or a device for dispensing the working fluid malfunctions, the heat generated by the heater may not be appropriately taken away by the working fluid of sufficient amount and flow rate, causing increase of the temperature of the heater continuously. In that event, the temperature of the working fluid sensed by the temperature sensor may not raise corresponding to the temperature of the heater so that the controller may have no chance to shut or cool down the heater, resulting in that the heater is liable to be damaged or burned out due to overheat.
In the aforesaid condition, the temperature of the heater raises very fast, especially for the heater having low mass and high power, such as an electric tungsten heater. Therefore, even though a further temperature sensor is disposed in the flow channel of the heating device, the temperature sensor in the flow channel might respond too slow so that the controller may not be able to stop the heater from heating in time before the heater is damaged or burned out due to overheat.
The inventors of the present invention believe and confirm that if the heating device uses a photodetector such as a photodiode to detect the temperature of the heater, the above-mentioned problem can be resolved because of high responding speed of the photodetector. Using the electric signal generated by a photodetector receiving lights from a light source to measure the temperature of the light source is well known for a person skilled in the art. Many applications in this regard have been developed. For example, a related application is disclosed in US Patent Pub. No. US2011/0008737 A1. However, the photodetector is liable to be damaged in high-temperature-variation circumstances. Therefore, in prior arts using a photodetector to detect temperature, instead of being disposed around the light source under detection, the photodetector is spaced away from the light source by a light guider such as a lens, a quartz rod and so on, and receives the lights from the light source through the light guider. In order that the lights from the light source can be transmitted to the photodetector properly, the photodetector is arranged in such a way that the light-receiving portion of the photodetector directly faces the light source through the light guider. Such arrangement may occupy much space, so that it is inapplicable to small sized heating devices due to the difficulty in spatial arrangement.